Thermal induced crystalline transition of bio-based polyamide 56

Thermal induced crystalline transition of polyamide 56 (PA56) was investigated by DSC, in-situ WAXD, and in-situ FTIR spectroscopy. The PA56 α-like phase, crystallized from formic acid solution, showed a Brill transition that completed at 200 °C (TB) and only γ phase crystallites were mainly obtained when cooling the PA56 samples to room temperature from molten state. The characteristic absorbance bands of PA56 α-like and γ phases on FTIR spectra were assigned. The strength of the hydrogen bonds in PA56 γ phase was stronger than those in α-like phase, which could be the main reason for the irreversible Brill transition behavior of the PA56 α-like phase. In-situ FTIR results indicated that the intermolecular hydrogen bond strength of α-like phase reduced during Brill transition before 200 °C upon heating. During the melting process, the intermolecular hydrogen bond strength first enhanced and then reduced. The Brill transition of PA56 upon heating was further confirmed by the decrease and increase in the intensity of the band at 1631 cm−1 and 1637 cm−1, corresponding to the vibration band (amide I) of CO group of α-like and γ phases of PA56, respectively. FTIR results also confirmed the deformation, twisting, and rocking vibrations of CH2 groups were enhanced during the Brill transition procedure.